Most gas-phase species possess strong, fundamental vibrational modes in the mid-IR bands. This necessitates the development of tunable high power, compact, low-cost continuous wave (CW) mid-IR laser sources for numerous applications such as chemical and environmental monitoring, medical diagnostics, atmospheric transmission measurements, as well as military and security applications. Commonly used to-date CW sources of coherent mid-IR radiation include direct laser radiation devices (known as class ‘A’ laser sources) and sources the operation of which is based on nonlinear optical processes (referred to as class ‘B’ sources).
The development of class ‘A’ solid-state laser sources has been recently significantly advanced and includes quantum cascade (QC) lasers, rare-earth doped fiber lasers, inter-band cascade lasers (ICL) and type-II inter-band lasers, to name just a few. While these laser sources demonstrated promising performance across limited wavelength bands, at other wavelengths of interest their performance has been rather unsatisfactory due to some fundamental limitations.
The QC lasers, for example, showed promising performance in the wavelength range of 5-12 um. While operating in a CW mode, on the other hand, a QC laser converts up to 70% of the injected electrical power to heat, which has to be dissipated from the active region of the laser to enable the required room-temperature operation. Given that the area of the active region is approximately 100 μm2, an efficient solution to address such dissipation to enable the QC generation of high-power single-mode coherent light continues to present a real challenge. In addition, for wavelengths shorter than 5 μm (for example, in the range between about 3 μm and 5 μm), the smaller energy gap between the upper laser state and the continuum states above the quantum wells results in a higher probability of carrier leakage into the continuum states, causing poor operational performance of a QC laser at these wavelengths at room temperature.
High power fiber lasers are widely used in the range from about 1 μm to about 2 μm (and Ho+3-doped fiber laser devices have been developed to expand the emission wavelength towards 3 μm and achieve a near watt-level output power). At the same time, the performance of fiber lasers quickly degrades at wavelengths above 3 μm, even under cooling conditions. For wavelengths exceeding about 3.2 μm, the maximum output power obtained from the Ho+3-doped fiber laser, for example, does not exceed the mW range. Similarly, while a laser source employing direct bandgap III-V semiconductors (for example, InGaSb/GaSb based materials) can operate in the 1.9-2.7 μm range at room temperature, and an exemplary room-temperature operation of a Sb-based semiconductor laser with output power of 80 mW was demonstrated at wavelengths up to 3 μm, the valence-band leakage and large Auger recombination significantly reduce, as a rule, the efficiency of operation at wavelengths above 2.8 μm. Another type of ‘A’ class laser—the vibronic solid-state laser—possesses broad gain bandwidths caused by phonon interaction. Sources utilizing Cr2+ or Fe2+-cations doped into II-VI compounds demonstrated laser emission in the range of 2 μm-3 μm. While a chalcogenide ceramic laser based on Cr2+:ZnSe can produce high output power in a single longitudinal mode, both thermal lensing and quenching from multi-phonon emission remain among factors principally limiting the ability to scale the power output.
Class ‘B’ laser sources—in particular those employing difference frequency generation (DFG) to produce coherent mid-IR emission in a very broad wavelength band at room temperature—are commonly used as well. The recognized shortcoming of the majority of DFG-based lasers is their bulky structure and substantial dimensions, which stem from a need for a laser pump source (such as, for example, a Ti-Sapphire pump laser) producing high power, single mode emission. The diode-laser-pump-based alternative of a DFB class ‘B’ laser, on the other hand, does not produce yet a sufficiently high-power output (which is currently limited to about 10 mW) due to the fact that the output power of a single mode diode laser is typically below 1 W.
Optically-pumped vertical external-cavity surface emitting lasers (VECSELs) employing various III-V materials, have been subject to research in recent years and shown to provide a flexible high-brightness high-power output laser platform for generation of light in visible-IR wavelength bands. The major advantage of a VECSEL is that it utilizes a semiconductor quantum-well gain structure that opens a possibility to tailor the output spectrum of a VECSEL by means of band-gap engineering to provide specific solutions to a variety of applications in the near infrared. For example, VECSELs operating at different wavelengths between 670 nm and 2.8 um have been discussed in literature. In particular, InGaAs/GaAs strained quantum wells have been extensively researched and are capable of spanning the wavelength range from ˜900 nm to 1200 nm. The open cavity design of a VECSEL provides access to the high intracavity power, which allows for wavelength tuning, linewidth control, and efficient intracavity nonlinear frequency conversion for not just single frequency operation, but also high-power non-linear wavelength generation covering a range of wavelengths from the UV to the far IR regions of the spectrum (see, for example, M. Scheller et al., in Optics Express, v. 18, 21112, 2010; or S. Kaspar et al., in Applied Phys. Letts., v. 100, 031109, 2012). A VECSEL laser operating at two different wavelengths is of interest in a range of applications including free-space wavelength-multiplexed optical communications as well as for optical distribution and generation of radar local oscillators and for nonlinear frequency generation of radiation from mid-IR up to THz frequencies for remote sensing applications (L. Fan et al., in Appl. Phys. Letts., v. 90, 181124, 2007).
Although VECSELs utilizing multiple cavities, intra-cavity etalons, spatial mode splitting, or a multiple-quantum-well-based medium have been shown to generate light at two wavelengths, all VECSEL devices of related art lack the degree of tunability and efficiency of a single wavelength VECSEL source. In particular, the need for a VECSEL system structured to generate simultaneous light outputs at multiple wavelengths that are independently tunable and not limited by any particular mutual relationship (describing, for example, a limitation imposed on a characteristic of light at a first wavelength by light at a second wavelength) has not been addressed to date.